leetcode-solution/Java/Solution107.java at main · e-chocolate/leetcode-solution · GitHub

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution107 {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        LinkedList<List<Integer>> res = new LinkedList<>();
        Queue<TreeNode> queue = new ArrayDeque<>();
        if (root != null) queue.offer(root);
        while(queue.size() != 0) {
            int n = queue.size();
            List<Integer> ans = new ArrayList<>();
            for (int i = 0; i < n; i++) {
                TreeNode t = queue.poll();
                ans.add(t.val);
                if (t.left != null) queue.offer(t.left);
                if (t.right != null) queue.offer(t.right);
            }
            res.addFirst(ans);
        }
        return res;
    }
}